Paint is a mixture of pigments, binders, extenders, additives and solvents. The paint should improve the appearance of and give protection to a substrate/base. The binder of the paint gives cohesion in the paint film and adhesion to the base. Fatty acid modified polyester, alkyd, is often used as binder in paint and lacquer. The air drying binder cures by reaction with oxygen in the air, where the double bonds from the fatty acids are the reaction point of the cross-binding reaction. Alkyd technology has some advantages and disadvantages compared to other binder technology:                The raw materials largely come from renewable sources.        The raw materials are relatively cheap.        Alkyd paint is easy to use, since it is a 1-component paint.        Relatively good technical properties such as scratch resistance, flexibility and wetting of the substrate. The scratch resistance is however not as good as for a 2-component paint and lacquer.        Alkyd paint dries relatively slowly compared to other types of paint technologies. In the curing phase the paint film is vulnerable for capture of dirt, as it is then sticky. This phase is longer for alkyd paints than for other types of paint.        The technology is available for water dilutable as well as solvent dilutable systems.        Alkyd paint is vulnerable to UV-degradation.        
EU has come with a VOC directive (Volatile Organic Compounds). This directive gives restrictions to how much organic solvent can be used in paint. The directive gives requirements in two steps, where the first is in force from 2007 and the second from 2010. In some categories, the requirement from 2010 is particularly demanding. Solvent diluted alkyd paint is encompassed with a requirement of maximum 300 g VOC per liter paint. In order to satisfy these new requirements, the paints must be reformulated. One option is to use a new binder so that the paint may be diluted with less solvent, without significantly increasing the viscosity. If such a reformulation is done without any other measures, the properties of traditional alkyd paints are dramatically changed, to the worse. In particular the curing time is considerably extended. For outdoors protection of wooden materials, solvent diluted alkyd paints are typically much used in Norway. A change to exclusively water dilutable systems is difficult in Norway, due to the climate. Thus, it is important to come up with technically good solutions for this type of technology.
Much work has been done in order to improve the alkyd technology, in particular with the aim to improve the UV-resistance, achieve faster curing time and a harder and more scratch resistant paint film. A part may be gained by the proper choice of monomers to the alkyd, some more may be gained by a good polymerisation process. Larger improvements require the preparation of an alkyd hybrid. This implies combining alkyd technology with other binder technologies. It may be relevant with a chemical modification of the alkyd polymer, most often a graft-copolymer. The most common chemical compounds used for this purpose are: poly(meth)acryl, silicone/siloxane or polyurethane. This improves the alkyd technology rather much, improvements which would not possibly be achieved without combining different technologies. Even though these chemical modifications are made, it is still a 1-component air drying system. The price of the new hybrids will be higher due to more expensive raw materials and more complicated processes. At least one extra production step must be accounted for.
Another possible method for improving alkyd paints is to mix in inorganic nanoparticles. First and foremost, the nanoparticles contribute to increased hardness and improved scratch resistance. It is assumed that the hardness of the particles themselves contribute to this. Today, paint consists much of fillers and pigments, which are mineralic. They are large, that is, the particle diameter is from 200 nm-500 μm. Typically, nanoparticles used in paint and lacquer have diameters from 10 nm-60 nm. They are in a completely different range of sizes, where the surface area of the particles gets very large in relation to the particle volume. This implies that nanoparticles may contribute to a completely different improvement, than what is achieved when admixing regular pigment particles even though the chemistry may be quite similar. The surface of the paint film gets smoother with nanoparticles, a smooth surface is beneficial in order to have better scratch resistance. The nanoparticles do not scatter the light so that the optical properties of the paint are not altered. The smaller particles, the larger surface and this is favourable in order to make a dense and hard film. The interaction between polymer and particle gets better.
Many nanoparticles for paint and lacquer have come to the market, for both water diluted and solvent diluted systems. The inorganic particle is hard and contributes to improved mechanical properties of the product. It has been found that the surface of the particles must be modified in order to improve the mixability with the binder. If this is not done, one may get problems with the storage stability or surface disturbances on the paint film. At best, only a neglectable improvement is achieved.
One method for preparation of surface modified nanoparticles is to react an already existing particle with a coupling reagent as disclosed in WO2006045713, WO2006125736 and US20040204521. The existing particles may be commercially available particles with size up to 1000 nm. The coupling reagent being used here, is an organic silicon compound with at least one alkoxy functionality. Selected chemical compounds may be chemically bound towards the coupling reagent before or after the coupling towards the nanoparticle. In this way, the surface modified nanoparticle may get additional functionality such as radical capturer, antioxidiser, UV-absorber, light stabiliser, flame retardant, photo initiator or combinations of these. The particles to be modified may be commercially available particles of SiO2 and Al2O3 or mixtures of these metal oxides. The use of such modified particles in thermoplastics, thermoset plastics, paint and lacquer is mentioned. Binder for air drying paint based on such surface modified nanoparticles is not disclosed.
WO2007020062 and WO2007020063 reports surface modification of existing metal oxide particles by using silane as coupling reagent. It is claimed protected a process which does not originate from a sol-gel synthesis, but which is based on dispersing particle agglomerates in an organic solvent. The aerosil-process or precipitation from solution may be processes for preparation of agglomerates of such nanoparticles. Immediately after dispersing, the particles are reacted with a silane-coupling reagent. Use of these particles in paint and lacquer is also reported, in particular, for increasing scratch resistance in fully hardened lacquer or paint. Binder for air drying paint based on such de-agglomerated and surface modified nanoparticles is not disclosed.
EP1526115 and US2007014917 reports preparation of organically modified nanoparticles in a sol-gel-process based on silicon tetrachloride, SiCl4. Remaining Si—Cl-groups may be reacted with R—OH, RNH2, R2NH, RPH2, R2PH, R—Mg—X, Li—R, where X is a halogen atom and R an organic residue or a silicon organic group. Modification giving particles with instauration or epoxy functionality in the R-group is mentioned, and use in paint and lacquer is reported. Binder for air drying paint based on surface modified nanoparticles is not disclosed.
EP1361245 and EP1359182 discloses amine functional polysiloxanes which may be used in epoxy systems. Paint/lacquer with improved hardness, better shine resistance and weatherability may be achieved. The disclosed polysiloxanes has a linear and not a particulate structure. Hence they may not be denoted as nanoparticles. Use of such siloxanes as binder for air drying paint is not reported.
In WO2006008120 it is disclosed water based dispersions of inorganic nanoparticles where the nanoparticles are surrounded by pure organic polymers. The organic polymers are often used in coatings, also without nanoparticles. It is reported use of the nanoparticles which are surrounded by pure organic polymers in air drying coating. However, the organic polymers are not bound towards the inorganic nanoparticle with a covalent chemical bond. Therefore, such nanoparticles which are surrounded by pure organic polymers, must be regarded as a physical mixture, and not as a hybrid matter where organic branches are chemically bound towards inorganic particles.
In US 2008/0017071 it is disclosed water based binder dispersions comprising three components: a) binders known per se, such as saturated or unsaturated polyester, polyurethane, alkyd resin, phenol resin, urea resin and melamine resin, b) inorganic nanoparticles, c) polymer particles of nano size. Component c) is present dispersed in water, and it covers component b). In order to improve the contact between component b) and c), the surface of the inorganic nanoparticles (component b), may be modified with suitable organic compounds. It is not disclosed how component c), when it covers component b), may be used as binder for air drying paint. Component c), when it covers component b), must for this be air drying per se.
In U.S. Pat. No. 6,750,270 it is disclosed a binder where nanoparticles and a binder known per se carries complementary reactive groups which may react chemically with each other during the preparation of the new binder. It is not mentioned or disclosed uses where the known binder per se is air drying. It is also not mentioned or disclosed that the new binder may be used as binder for air drying paint.
WO2005100450 discloses a process for preparation of different types of polybranched inorganic/organic hybrid polymers. The process is based on a chemical reaction between one amine group in a polybranched inorganic/organic hybrid polymer and one suitable mono functional chemical compound. It is disclosed one type of water dilutable inorganic/organic hybrid polymer with cross-binding acryl groups. Water-containing solutions or dispersions of this type of hybrid polymer are obviously alkaline due to its chemical construction as polyamine, and thereby poorly suitable as binder for air drying paint.
WO2007053024 discloses a process for preparation of polybranched inorganic/organic hybrid polymer with UV-absorbing properties. Use as UV-absorbing component in binder-, lacquer- and coating products is reported. On the other hand this type of polybranched inorganic/organic hybrid polymer is very poorly suitable as binder for air drying paint.
Purpose
It is a purpose of the present invention to provide binders for air drying paint which may impart short drying times to the paint, and at the same time low viscosity at low content of VOC.
Further it is a purpose to provide processes for preparation of binders as given above.
Further it is a purpose to provide air drying paint or lacquer, comprising binders as given above.
The Invention
The present invention provides a binder for air drying paint, based on metal oxide-containing nanoparticles with unsaturated organic branches, where at least one metal atom of the nanoparticle is chemically bound to an organic branch with at least one unsaturated ethylenic functionality.
A second aspect of the invention is a process for preparation of binder according to the invention, where it in a first step, is prepared a polybranched organic/inorganic hybrid polymer of hydrolysable metal compounds with functional amine groups. This reaction is carried out by controlled hydrolysis and condensation after water is mixed with the hydrolysable metal compounds (Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. C. Jeffrey Brinker and Geirge W. Scherer, Academic Press Inc., New York, 1990). Such a process is often referred to as a sol-gel process. The process comprises at least one further process step, in which one or more of said functional amine groups is/are reacted with at least one unsaturated organic compound, so that it is formed a covalent chemical bond or ionic bond between the N-atom of the functional amine group and at least one atom of the unsaturated organic compound.
A third aspect of the invention is air drying paint or lacquer containing at least one binder according to the invention.
A nanoparticle may be defined as a solid particle having a size between 1 nm and 100 nm.
The quality of a binder for air drying paint is, among others, dependant on the number of cross-binding groups being linked together on a chemical base-structure. The number of cross-binding groups in a binder is in the following denoted with fkryss. Generally seen, a binder with larger fkryss will form a better cross-bonded network, show more wear- and weather resistance and dry faster than a comparable binder with smaller fkryss. At the same time, the molar weight of the binder is normally larger if fkryss is larger. This may imply undesirably high viscosity and/or undesirably extensive use of solvents and thereby VOC.
One possibility in order to achieve large fkryss with low contribution to viscosity is to prepare binders with a compact structure, based on a core of metal oxide with cross-binding organic branches. Such binders may be denoted as “inorganic/organic hybrid binders” or quite simply “hybrid binders” because they consist of an inorganic core (metal oxide) and organic branches on the surface. Due to their particulate nature they may also be denoted as nanobinder.